A threaded joint has been popularly used to connect steel pipes used in a petroleum installation for oil industry such as oil country tubular goods. In connecting steel pipes used for search and production of oil or gas, conventionally, a standard threaded joint which is stipulated in API (American Petroleum Institute) standard has been typically used. However, recently, deepening of a well for crude oil or a natural gas has progressed and the number of horizontal wells and directional wells instead of vertical wells has been increasing and, hence, the drilling and production environment has become difficult. Further, the number of wells developed in an appalling circumstance such as oceans and polar regions has been increasing and, hence, the performances which threaded joints have to achieve are diversified including compression resistance, bending resistance and sealability against external pressure (external pressure resistance). In view of the above, the number of instances where a special threaded joint having high performance referred to as “premium joint” is being used have increased, and a demand for the improvement of performances of the premium joint has been also increased more and more.
A premium joint is a coupling-type joint where externally-threaded members (hereinafter referred to as “pins”) each of which includes a tapered thread, a seal portion (to be more specific, a metal to metal seal portion) and a shoulder (to be more specific, a torque shoulder) and is formed on a pipe end portion, and an internally-threaded member which includes tapered threads, seal portions (to be more specific, metal to metal seal portions) and shoulders (to be more specific, torque shoulders) and connects the pins to each other (hereinafter referred to as “box”) are jointed to each other. The tapered threads are important to firmly fix the pipe joint, the seal portions play a role of ensuring gas tightness by bringing the box and the pins into metal contact at such portions, and the shoulders form shoulder faces which play a role of abutments during make-up of the joint.
FIGS. 3A-3C are schematic explanatory views showing a conventional example of a premium joint for an oil country tubular goods use. FIGS. 3A-3C also include longitudinal cross-sectional views of a threaded joint of a circular pipe (a cross-sectional view where a pipe axis extends in the cross section). The threaded joint includes pins 3 and a box 1 corresponding to the pins 3. On an outer surface thereof, the pin 3 has an externally-threaded portion 7 and a nose (also referred to as a pin nose) 8 which is an elongated portion formed adjacent to the externally-threaded portion 7 on each distal end side of the pin 3 and has no threads. The nose 8 has a seal portion (to be more specific, a metal to metal seal portion) 11 on an outer peripheral surface thereof, and a shoulder 12 on an end surface thereof. The box 1 corresponding to the pins 3 has internally-threaded portions 5, seal portions 13 and shoulders 14 on an inner surface thereof, and these portions are portions being threadedly engaged with or brought into contact with the externally-threaded portions 7, the seal portions 11 and the shoulders 12 of the pins 3 respectively.
In the conventional example shown in FIGS. 3A-3C, the seal portion 11 is formed on a distal end portion of the pin 3, and desired seal performance can be realized by imparting an appropriate make up torque. However, the make up torque is influenced by a lubrication condition, a surface quality and the like. As a design of a threaded joint which does not largely depend on these factors, there has been known a radial-directional seal method (also referred to as a radial seal type) where a radial directional component of a seal contact pressure is relatively increased.
With respect to the point that the seal portion is provided at a portion different from the shoulder, the radial-seal-type threaded joint is substantially equal to the threaded joint shown in FIGS. 3A-3C having the seal portion on the distal end portion of the pin.
When the shoulder faces which play roles of stoppers are brought into contact with each other during make-up of the threaded joint, a contact reaction force in the axis direction acts on a pin pipe end side so that a maximum axial reaction force is generated on a load flank face of a threaded portion on a pin pipe end side whereby galling is liable to occur.
JP 2001-21072 A discloses a technique of preventing galling at a box end portion by decreasing a contact reaction force at the box end portion. However, a contact reaction force on a box center side, that is, on a pin pipe end side is kept high. Hence, JP '072 cannot overcome the galling on the pin pipe end side. In view of the above, it is possible that a contact reaction force can be decreased by applying the technique of JP '072 that a thread gap (a face to face gap formed between opposedly-facing stabbing flank faces) is changed by changing a pitch in the axis direction in thread cutting also to a pin pipe end side. However, the thread gap is extremely small in the first place and there are variations in pitch in the axis direction. Hence, it is difficult to industrially carry out the fine adjustment of the thread gap.
There is also the drawback that it is difficult to provide a threaded joint for an oil country tubular goods having excellent galling resistance at a threaded portion on a pin pipe end side as industrial products.